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In 2003 a particle temporarily called X(3872), by the Belle experiment in Japan, was proposed to be a tetraquark candidate,[1] as originally theorized.[2] The name X is a temporary name, indicating that there are still some questions about its properties to be tested. The number following is the mass of the particle in 7000100000000000000♠100MeV/c2.

In 2004, the DsJ(2632) state seen in Fermilab's SELEX was suggested as a possible tetraquark candidate.[citation needed]

In 2007, Belle announced the observation of the Z(4430) state, a ccdu tetraquark candidate. In 2014, the Large Hadron Collider experiment LHCb confirmed this resonance with a significance of over 13.9σ.[3][4] There are also indications that the Y(4660), also discovered by Belle in 2007, could be a tetraquark state.[5]

In 2009, Fermilab announced that they have discovered a particle temporarily called Y(4140), which may also be a tetraquark.[6]